The present invention relates to rechargeable galvanic lithium cells having a negative electrode, a positive electrode and an electrolyte which is comprised of an alkali-metal salt dissolved in a mixture with a plurality of organic solvents or organic polymers.
Because of the high reactivity of the alkali metal, lithium cells contain electrolytes based exclusively on nonaqueous organic solvents. Such electrolytes generally include a conducting salt dissolved in a solvent such as propylene carbonate, ethylene carbonate, tetrahydrofuran, dimethoxyethane, .gamma.-butyrolactone or the like, and also of mixtures of these components.
The conducting salts often used are lithium salts such as LiAsF.sub.6, LiClO.sub.4, LiBF.sub.4, LiSbF.sub.6, LiAlCl.sub.4 or LiCF.sub.3 SO.sub.3. Their suitability for a particular use depends upon a good dissociation capability in the particular solvent, so that a sufficient number of ionic charge carriers are present for electrolytic current conduction. The conductivity of good organic electrolytes should be at least on an order of magnitude of 10.sup.-3 S .times.cm.sup.-1 .
As a further condition, the dissolved salts in the galvanic lithium cell (like the solvents) must be nontoxic substances, and must be stable, both thermally and electrochemically with respect to the aggressive lithium (for potentials ranging from 0 to 4 V, against Li/Li.sup.+).
In this respect, even well established and proven conducting salts still leave something to be desired. There is a risk of detonation in the case of perchlorates. According to A. Dominey et al. (Proc. Intersoc. Energy Convers. Eng. Conf. 1990 (25), 3, 382-4), other salts containing a complexed fluoride anion are susceptible to reduction of the central ion if this has an appropriate metallic nature. This can occur, for example, in the case of SbF.sub.6.sup.-, whereas anions such as BF.sub.4.sup.-, PF.sub.6.sup.- or AsF.sub.6.sup.- (having a nonmetal as the central ion) are thermally unstable. Although lithium trifluoromethanesulphonate (LiCF.sub.3 SO.sub.3) is stable, it does not provide optimum conductivity as a conducting salt.
As a suitable conducting salt for rechargeable lithium batteries containing polymeric electrolytes, Dominey (cited above) mentions lithium tris(trifluoromethanesulphonyl) methanide, LiC(CF.sub.3 SO.sub.2).sub.3, which is thermally stable and which is said to exhibit a maximum conductivity (in a polymeric electrolyte) of 5.times.10.sup.-4 (ohm.times.cm).sup.-1 at 25.degree. C. As a comparison, 1.times.10.sup.-2 (ohm.times.cm).sup.-1 at 25.degree. C. is mentioned as the conductivity for a 1.0 molar solution of this salt in tetrahydrofuran. The free acid, HC(CF.sub.3 SO.sub.2).sub.3, on which this trisubstituted methane compound is based was first prepared by L. Turowsky and K. Seppelt (Inorg. Chem. 27, 2135-2137 (1988)).
PCT Application WO92/02966 proposes using fluorinated "methane salts" in nonaqueous battery electrolytes. These components are theoretically derived from a more simply constructed "acid", tris(fluorosulphonyl)methane, HC(SO.sub.2 F).sub.3, by replacing the three F atoms by halogenated alkyl groups and the H atom by a monovalent metal cation (Me.sup.+). This acid was first synthesized by G. Kloter, H. Pritzkow and K. Seppelt (Angew. Chem. 1980, 92, 954).